Abomasal Impaction
David Francoz
Definition and Classification
Abomasal impaction is the accumulation of firm ingesta in the abomasum with failure of aboral transport associated with an increased volume of the abomasum.
The impaction involves the body and the pyloric antrum of the abomasum. However, impaction of the pyloric antrum only is also described in dairy cattle.1 Abomasal impaction can be classified as primary when no underlying cause can be identified and as secondary when impaction is secondary to another condition. Dietary abomasal impaction refers to abomasal impaction secondary to ingestion of abnormal diet.Etiology and Pathophysiology
Primary abomasal impaction is considered idiopathic and has been described mainly in dairy cows in the early postpartum period.1 Dietary abomasal impaction is the result of feeding poor-quality, coarse roughage as the sole feed. Abomasal impaction can also be secondary to consumption of nonfood material such as sand or gravel or secondary to abnormal alimentary behavior or ingestion of feed mixed with sand.2-5 Secondary abomasal distention may also occur with normal diets after correction of abomasal volvulus, as a result of reticuloperitonitis, or as a result of the development of adhesions between the abomasum and the rumen or the abdomen wall (or both). Lymphoma involving the abomasum or other space-occupying lesions adjacent to the pylorus may lead to abomasal distention. These conditions are referred to as chronic indigestion and are discussed elsewhere in this chapter. Abomasal emptying defects of Suffolk sheep are addressed in the previous section.
The pathophysiologic mechanism of abomasal impaction is not known. In dietary abomasal impaction, animals fed roughage that is poorly digestible and incapable of meeting their energy requirements consume as much as the rumen will physically allow.
The flow of ingesta from the forestomachs to the abomasum normally contains only small, finely digested particles of forage material. With chronic engorgement of highly lignified, poorly digestible forage, larger particles escape the forestomach and accumulate in the abomasum. Once a mass of fiber forms in the abomasum, further accumulation of particulate material is enhanced. Accumulation of dry materials modifies the abomasal content viscosity and consequently, the abomasal emptying rate decrease.1 Additional ingesta distends the abomasum to several times normal size, which leads to a severe abomasal distention that prevents any return to normal motility. Also, abomasal secretion may be inhibited by the cachexia and chronic distention of the organ.Clinical Signs and Differential Diagnosis
Adult dairy cows with primary abomasal impaction have a consistently decreased appetite, but other signs are inconsistent.1 Primary abomasal impaction should be considered in the differential diagnosis of nonspecific clinical signs of decreased milk production and appetite in dairy cows.1 Conditions that cause dehydration and bilateral abdominal distention with absence of feces must be considered in the differential diagnosis. In many of these cases, exploratory celiotomy is necessary to arrive at a definitive diagnosis.
Dietary abomasal impaction has mainly been described in beef cows, but dairy cows exposed to similar feeding and environmental conditions may also be affected. Several animals in a herd may be affected over a short period. Beef cows with abomasal impaction develop abomasal and ruminal distention over a period of days to weeks. Close monitoring of animals reveals reduced feed intake and a reduced volume of firmer than normal feces. The animal may have right ventral abdominal enlargement. Ruminal contractions are of normal or increased frequency but are often reduced in amplitude. In the later stages of the disease, ruminal motility is often absent.
Cattle with advanced abomasal impaction may be recumbent and groan with each respiration. The consistency of the ruminal ingesta, as judged by ballottement, may be more fluid than expected on the basis of the coarse diet. Other animals have a uniformly firm and distended rumen. The pulse and respiration are usually normal until the animal is near death, at which time tachycardia develops. The abomasum may be palpable as a firm mass behind the right coastal arch. Rectal examination reveals a distended rumen; often the ventral sac extends to the right body wall (L-shaped rumen). The pyloric part of the distended abomasum may be palpable in the right ventral quadrant. Wintering beef cows usually are pregnant, and therefore the uterus may prevent palpation of the abomasum. Feces are absent or very scant and dry. Mucus may be all that clings to the clinician’s sleeve after rectal examination. The body condition of affected animals is invariably poor because negative energy balance precedes and is amplified by the impaction. If the abomasum ruptures, signs of generalized peritonitis occur. Death usually follows within hours of rupture.Ancillary Tests
The hypochloremic, metabolic alkalosis typical of upper gastrointestinal obstruction in ruminants does not always develop in abomasal impaction.1 Initially some fluid ingesta may pass through the abomasum, preventing chloride sequestration. However, some cattle have metabolic alkalosis, with chloride accumulating in the rumen to a level greater than 30 mEq/L (internal vomiting). In terminal stages, a mixed acid-base imbalance with both a metabolic alkalosis and a metabolic acidosis can result in a normal overall pH. Anemia and leukopenia can accompany the cachexia of chronic abomasal impaction in poorly fed animals. If abomasal rupture has occurred, profound hemoconcentration and leukopenia are present. Abdominocentesis generally is not useful in diagnosing abomasal impaction because peritoneal fluid is abnormal only after abomasal rupture.
Treatment
Medical management includes correction of fluid and electrolyte abnormalities. Early cases may be resolved with easily digestible feeds, aggressive fluid therapy, and oral administration of laxatives such as mineral oil (4 L daily). The use of prokinetic drugs may be indicated in order to increase abomasal emptying rate and the passage of ingesta through the pylorus. However, no data on the efficacy of the administration of prokinetic drugs in the specific treatment of abomasal impaction are available. Constable and colleagues3 published a comprehensive review on the data available for the recommendation of the use of prokinetic drugs in abomasal disorders. They concluded that (1) erythromycin (8.8 to 10 mg/kg IM bid) is the most effective drug to increase abomasal emptying rate; (2) according to in vitro studies, bethanechol (0.07 mg/kg SC tid) is potentially effective, but no controlled clinical trials have actually demonstrated its effectiveness; and (3) metoclopramide is ineffective in promoting abomasal emptying rate in cattle and cannot be recommended. However, prudent use guidelines from many countries indicate that the use of an antibiotic such as erythromycin for its prokinetic activities is controversial and should not be recommended. Pregnancy may be terminated by induction of parturition with corticosteroids or prostaglandin, or both, which improves comfort.
Different approaches have been proposed for a surgical treatment of abomasal impaction. Rumenotomy, followed by installation of a nasogastric tube inserted into the abomasum through the omasum, can be performed. Through the indwelling tube, laxatives and emulsifiers may be given during the postoperative days to aid in softening and removing the abomasal contents. Mineral oil (8 mL/kg/day), dioctyl sodium sulfosuccinate (50 mg/kg/day), magnesium hydroxide (1 g/kg/ day), and magnesium sulfate (2.5 g/kg/day) have each been recommended. However, none of these treatments has been evaluated, and detrimental effects have been reported with some of them (see discussion of magnesium hydroxide in the Acute Abdomen in Ruminants section).
Right paralumbar fossa, right paracostal, or right paramedial approaches can also be used to gain direct access to the abomasum. The authors prefer the right paracostal approach, with the animal in left lateral recumbency, for abomasotomy and removal of abomasum contents. However, even with this approach, complete exteriorization of the abomasum and exploration of the oral portion of the abomasum and omasum is often not possible, particularly in large animals. Even if the prognosis for restoring the abomasal function is poor, the authors have had some positive outcome, depending on the cause of impaction and the extent of abomasal lesions. External massage may help break up the contents of the abomasum. Intraluminal administration of 5% dioctyl sulfosuccinate solution or saline can also be performed during surgery.1Prognosis
Because most cases of abomasal impaction are quite advanced when brought to the attention of the veterinarian, treatment is usually unrewarding. In dairy cows the short-term prognosis was reported to be good (93%) for impaction that affected only the pyloric antrum and guarded (50%) for impaction involving the entire abomasum.1 The clinician must weigh the severity of the metabolic disturbances and the likelihood of recovery. Salvage by slaughter is often the most economic recommendation. If therapeutic measures do not resolve the impaction, death usually occurs within a few days of the onset of severe signs.
Necropsy Findings
Emaciation and a firm, grossly enlarged abomasum are consistent with primary or dietary abomasal impactions that involve the entire abomasum. The abomasal contents resemble normal, dry ruminal contents. The rumen is also enlarged but either filled with homogeneous, watery ingesta that lack normal stratification or impacted with dry ingesta, like the abomasum. Accumulation of sand or gravel is evident.
Prevention and Control
Prevention of dietary abomasal impaction requires proper dietary management of cattle in cold weather. Because animals outside without shelter have substantially increased maintenance energy requirements in cold, windy weather, straw or corn stover (stalks) is not adequate as the sole feed. Concentrates and better-quality forage prevent abomasal impaction. Monitoring body condition during winter weather alerts the good manager that supplemental feed is needed before abomasal impaction occurs.